Leif,
if Iyou had to determine why letterspacing as e m p h a s i s was ever
invented, then the absence of italic in Fraktur definitely seem to have
been a motivation (I'm not going to track this down right now, however I
seem to recall use of headers with darker type color, presumably because
they were in a bold font, so while I'm sure that italics were missing,
I'm not sure about bold).
In German, this use of letterspacing as emphasis definitely carried over
into typewritten text, and, at least in some demographics, is alive and
well in the internet age (I keep seeing it in sans-serif text, with
italics/bold readily available in the UI, yet they use letterspacing).
Another reason that Fraktur could use letterspacing for emphasis is
because it's such a narrow type style. It makes typefitting in narrow
columns rather easy (if you use hyphenation - German with its compound
nouns really requires hyphenation, there's no way you can adjust lines
well without it, so there's not that resistance to it that I see at
times in English).
As a result, you have readers who never (=hardly ever) see letter
spacing used in justification, yet are familiar with it for emphasis.
If, in modern, "roman" text, you suddenly were to enable letter-spacing
for very narrow columns, the resulting, very noticable variation in
letterspacing would be interpreted as "ransom note".
What I'm driving at with this example is that the acceptability of
certain styles of layout (hyphenation, letterspacing for justification,
letterspacing for emphasis) are definitely not associated with the
script (and not associated, as someone helpfully suggested, with the
element) but with local conventions (language and or region).
Short of an effort like CLDR you can't possibly hope to collect all
possible information on permissible usage of these layout styles.
However, you can make sure that whatever document you create does not
suggest that there's a simple classification of scripts (or typestyles)
that will somehow contain the right answers.
A./
PS: on hyphenation the Unicode forum (http://unicode.org/forum) just
recently gave an exception to the dictum "Arabic is never hyphenated"
(turns out to be false on the script level, but true on the language
level. Uighur, written in Arabic script, can apparently be hyphenated).
PPS: incidentally, as an aside, Fraktur used a distinction that's hard
to map to modern usage: certain foreign/Latin words were set in Antiqua
by convention. This is similar, but not equivalent, to the modern
English practice of using italics for some foreign/Latin words. (The set
of words so treated was different than modern usage, hence it couldn't
be mapped on the rendering level.)